1. Field of the Invention
The present invention generally relates to a method and apparatus for establishing a net datum position of a framing structure. More specifically, this invention relates to a method and apparatus that finds reference surfaces of an automotive vehicle frame that has been assembled within an acceptable tolerance range, and despite the within-tolerance variation of the reference surfaces on the frame, the frame is immobilized in its free state so that work can be subsequently performed on the frame to fabricate a net datum locating feature in its original design-intent location on the frame.
2. Description of the Related Art
In the manufacturing of automobiles and trucks, a chassis frame typically includes an underbody, a pair of side frames, and front and rear headers wherein such chassis frame usually undergoes a progressive series of positioning and welding steps before a rigid chassis frame is produced. Though vehicle bodies are still manually assembled and welded, emphasis on automated assembly and welding operations has generated numerous automated and semi-automated framing systems.
Generally, in such prior art processes it is a common object of a framing system to accurately locate the body components relative to each other and maintain such location throughout later welding operations, until the structural rigidity of the body is sufficient to preserve the desired geometric configuration throughout the assembly line. A further object for most framing systems is to provide sufficient flexibility to accommodate more than one body style, so as to enhance the versatility of the assembly line and reduce the tooling cost per assembly unit.
Chassis frames, however, have not received as much attention as vehicle bodies with respect to automated assembly techniques and accordingly, are currently still being assembled on fixtures built to design-intent, wherein a pair of rail members are clamped in place with a series of cross members therebetween and subsequently welded together to permanently fasten the rails and cross members.
Unfortunately, the heat generated from welding operations causes the chassis frame to warp or twist out of its design-intent geometry and position. Such warping is problematic because the individual components of the chassis frame include locating surfaces or holes that are relied upon for locating other features produced in subsequent operations. Unfortunately, due to warpage of the chassis frame from the welding operation, such locating surfaces or holes are intolerably displaced from their design-intent position, and thereby negatively effect the location of the other features produced during subsequent operations.
There are at least two well-known attempts to solve this problem. First, many manufacturers use a frame straightening process in an attempt to correct the warping. The frame straightening process, however, tends to be an exercise in futility since the process introduces additional built-in stress to the chassis frame and can lead to further displacement of the rails and cross member structure. Such frame straightening processes are rarely, if ever, capable of reliably straightening the entire chassis frame back into its exact design-intent geometry.
A second solution involves welding the components of the chassis frame together, then forcibly clamping the chassis frame from its free state back into its exact design-intent geometry, then producing holes and other features, and finally unclamping the chassis frame. This solution is performed with the expectation that the holes and other features will remain positioned within a certain tolerance relative to the design-intent geometry of the chassis frame when the part is unclamped. Unfortunately, however, the chassis frame tends to spring back to its warped free state when unclamped and thus, the holes and other features of the chassis frame are carried away from their design-intent position, in some cases out of the acceptable tolerance range.
In addition to the above-mentioned general approaches, several other locating or positioning devices associated with processing vehicle chassis frames have been proposed in the prior art. For example, U.S. Pat. No. 4,976,026 to Dacey, Jr. addresses the problems associated with locating a datum reference on a partially assembled structural object such as an automobile chassis frame. As a solution, Dacey, Jr. teaches an apparatus having a probe portion thereof for detecting pre-established surfaces and features on the vehicle body to establish a datum position. Tooling attached to the apparatus subsequently references the datum position established by the probe portion, and performs work on the vehicle body in relation thereto.
Dacey, Jr. discloses the apparatus as having a base rigidly mounted to a floor, and having a transfer platform attached to the base with guideposts to ensure freedom of movement of the transfer platform in a first horizontal direction toward the vehicle body. A support platform is attached to the transfer platform using guideposts to ensure freedom of movement relative thereto in a second horizontal direction that is perpendicular to the first horizontal direction. A vertical platform is attached to the support structure and is moveable therewith along guideposts in a vertical direction. A form and pierce device is attached to a lower portion of the vertical platform for performing work on the vehicle body. A pair of hydraulic work supports interpose each of the platforms, at clamping plate locations, for locking horizontal and vertical movement of the apparatus. A series of probes are attached to an upper end of the vertical platform for detecting the pre-established surfaces and features of the vehicle body from which the datum position is established. For example, one of the probes includes a tapered surface to permit the probe to climb vertically onto a fender rail, and another probe is tapered to guide it into a preformed orifice in the vehicle body. Once the probes locate the pre-established surfaces and features of the vehicle body, fluid pressure is introduced to the work supports so that push rods of the work supports exert an equal clamping force on either side of the positioning plates for locking the transfer, support, and vertical platforms in place. The datum position of the pre-established surfaces and features is thereby established to permit the form and pierce tool to perform work upon the chassis frame in relation to the datum position.
As discussed above, Dacey, Jr. provides an apparatus having a sub-assembly for forming and piercing a vehicle body panel or sub-frame in direct relation to a datum position established by detecting pre-established surfaces and features of the vehicle body by another sub-assembly of the same apparatus. Unfortunately, however, Dacey, Jr. requires that a preformed orifice and specific body panels be located on the vehicle body prior to immobilizing the tooling device for subsequent forming and/or piercing operations. Additionally, Dacey, Jr. does not teach or disclose that the apparatus is capable of rigidly immobilizing the entire vehicle body by itself, not just proximate the datum location, thereby enabling work to be performed anywhere on the vehicle body. Finally, the piercing operation disclosed in Dacey, Jr. is performed by a tooling device, relative to a datum location established by probing various predefined features that have pre-existing tolerance variation. The positions of the various predefined features on the vehicle body, such as the top of the fender rail and the preformed orifice, have size and locational tolerance variations. Further, the apparatus including the probes, platforms, and piercing tooling have size and locational tolerance variations also. Therefore, the piercing operation is performed relative to specific features of the vehicle body that have tolerance variation, rather than performed directly relative to a net datum position of the entire vehicle body frame.
U.S. Pat. No. 5,987,726 to Akeel identifies a general problem with assembling body panels, wherein internal stresses are built into body assemblies. Therefore, Akeel teaches the use of programmable body panel positioners for compliant positioning of body panels or tooling during assembly operations to achieve a stress-free vehicle body assembly. Akeel discloses a parallel link mechanism having a base plate, a locator plate spaced above the base plate, and three pairs of linear actuator links attached between the base and locator plates by universal joints. Either a tool or a body panel member is clamped to the locator plate for processing the body panel member. The actuators are screw drives for changing the length of each link in a corrective manner in response to variation in processing forces. A feedback control system, including force sensors and encoders, is communicated with the links so that the actuators react to processing forces for increasing the holding force on the body panel member for higher support rigidity, or to decrease the holding force for controlled compliance and a stress free assembly.
Unfortunately, Akeel involves use of very complex apparatus used to avoid building stresses into the body assembly. Additionally, clamping forces are nonetheless imparted to the body panels, albeit in an adjustable manner.
From the above, it can be appreciated that the general solutions of the prior art relative to frame warpage and out of position holes and features are not fully optimized, and no specific viable solutions have been identified in the prior art. Therefore, what is needed is a simple apparatus and method for finding a chassis frame despite within-tolerance variation in size, position, or geometry of the chassis frame, and immobilizing the chassis frame in its free state without imparting forces sufficient to displace the chassis frame, whereby a net datum position of the chassis frame is established so that net datum locating features can be subsequently produced in precise design-intent locations on the frame.
According to the present invention, there is provided an apparatus and method for finding and immobilizing a workpiece in its free state, within an acceptable tolerance range, to establish a net datum position of the workpiece without compressingly clamping any portion of the workpiece so as to avoid deforming or displacing the workpiece to avoid building stress into the workpiece or incurring springback of the workpiece. The apparatus includes a pair of opposed pads for xe2x80x9cfindingxe2x80x9d the workpiece by independently coming into contact with opposite reference surfaces of the workpiece. An advancing device, such as a work support, connects to each of the opposed pads for advancing the opposed pads from a retracted position to find an advanced position in contact with the opposite portions of the workpiece. The advancing device advances the pair of opposed pads into contact with the workpiece, regardless of the exact location of the reference surfaces on the workpiece, without measurably displacing the workpiece. A locking mechanism is provided for locking the pair of opposed pads in their advanced position, whereby the pair of opposed pads lock in place without imparting any force on the workpiece, so as to immobilize the workpiece therebetween so that a net datum position is established for the workpiece.
It is an object of the present invention to provide an apparatus and method that is capable of immobilizing a chassis frame or carriage and establishing a net datum position of such carriage for subsequent processing.
It is still another object to provide an apparatus and method that is adapted to immobilize a chassis frame or carriage in its free state as it rests upon a chassis frame fixture.
It is yet another object to provide an apparatus and method for finding the position of a chassis frame and immobilizing the chassis frame in its free state as it rests upon a fixture, that can be used in conjunction with a tool such as a form and pierce device, to produce features in precise locations on the chassis frame that are net to the fixture, and thus to design-intent, despite tolerance variations within preselected acceptable limits in size or location of the chassis frame.
It is a further object to provide an apparatus and method that finds the position of and immobilizes a chassis frame in its free state without imposing on the chassis frame external forces sufficient to measurably deform or displace the chassis frame from its free state, in order to establish a net datum position according to design-intent so that this net datum position can be used as a reference to perform subsequent work on the chassis frame after engagement of the apparatus.
It is yet a further object to provide an apparatus and method for finding and immobilizing a chassis frame in its free state at a series of different workstations to perform work on the chassis frame so as to establish a net datum position or location and use the net datum position throughout using common fixtures, to perform subsequent work on the chassis frame without subjecting the chassis frame to clamping forces that not only deform and distort the chassis frame, but that also vary from workstation to workstation. Thus, the different tools produce different features in precise locations on the chassis frame that are net to the common fixtures, and thus to design-intent, despite tolerance variations in size or location of the chassis frame, and that are not influenced by varying clamping forces.
These objects and other features, aspects, and advantages of this invention will be more apparent after a reading of the following detailed description, appended claims, and accompanying drawings.